In accordance with the prevalence of notebook-type personal computers, portable phones, digital cameras, and the like, demand for secondary batteries is increasing for driving these small-scale electronic apparatuses. Further, in these electronic apparatuses, use of non-aqueous electrolyte secondary batteries (in particular, lithium ion secondary batteries) is being developed because such non-aqueous electrolyte secondary batteries can be made to have a high capacity.
In addition to use of such non-aqueous electrolyte secondary batteries in small-scale electronic apparatuses, studies are made on application of the non-aqueous electrolyte secondary batteries to use in which a large electric power is demanded such as in vehicles (e.g., electric vehicles (EVs), hybrid vehicles (HVs), plug-in hybrid vehicles (PHVs)), power sources for use at home (e.g., a home energy management system (HEMS)), and the like. In this case, a large electric power is obtained by means such as increase in scale of electrode plates in the non-aqueous electrolyte secondary batteries, lamination of numerous electrode plates to form an electrode body, and assemblage of numerous battery cells to make an assembled battery.
Typically, in the non-aqueous electrolyte secondary batteries, an electrode body is formed by lamination in a state in which a separator is interposed between a positive electrode plate and a negative electrode plate, and the electrode body is housed in a case together with a non-aqueous electrolyte. The electrode plates are produced by applying a paste containing an electrode active substance on a surface of a current collector made of a metal plate (metal foil), drying, and molding into a predetermined shape.
A non-aqueous electrolyte secondary battery is disclosed in, for example, Patent Literature 1. Patent Literature 1 discloses a non-aqueous electrolyte secondary battery including an electrode body including a positive electrode and a negative electrode, and a non-aqueous electrolyte. In the non-aqueous electrolyte secondary battery, the positive electrode includes a positive electrode mixture layer (positive electrode active substance layer) containing a positive electrode active substance made of lithium transition metal oxide. An amount of water in the positive electrode mixture layer, as detected by Karl Fischer's method after the electrode is held at 120° C. for 30 minutes, is 300 ppm or less. An amount of water in the positive electrode mixture layer, as detected by Karl Fischer's method after the electrode is held at 300° C. for 30 minutes, is 3000 ppm or more and 10000 ppm or less. According to this non-aqueous electrolyte secondary battery, excellent battery characteristics and high reliability (endurance at the time of internal short-circuiting) are made compatible with each other by adjusting the amount of water contained in the electrode mixture layer to be within a suitable range.
However, in a conventional non-aqueous electrolyte secondary battery, the positive electrode contains water (the amount of water is within a range from 3000 ppm to 10000 ppm), and this water raises a problem of decrease in reliability of the non-aqueous electrolyte secondary battery. More specifically, there is a problem in that, during the use of the non-aqueous electrolyte secondary battery, water is released into the secondary battery for a long period of time, which causes generation of a gas for a long period of time or deterioration of battery performance.
[Patent Literature 1] JP 2014-10981-A